1. Field of the Invention
The present invention relates to a method for producing oxygen gas which includes compressing liquid oxygen obtained by cryogenic separation and then evaporating the liquid oxygen by heating to prepare higher pressure gaseous oxygen.
2. Description of the Related Art
A large amount of higher pressure gaseous oxygen is used in oxidizing refining steps in steel-producing converters in the steel industry, synthetic steps of ethylene oxide by oxidation of ethylene in the chemical industry, and partial oxidation steps of fuel, such as coal and petroleum residues, in fuel fired power plants. The demand for such oxygen has tended to increase in recent years.
A typical method for producing oxygen on an industrial scale is cryogenic separation, which includes rectification of raw air at low temperatures to separate out oxygen. In the cryogenic separation, nitrogen and oxygen are separated out from raw air by means of a difference in boiling point. That is, liquefied air is supplied to a rectifier, and nitrogen, having higher volatility than that of oxygen, is evaporated in the rectifier to yield a high concentration of liquid oxygen.
In a method for producing higher pressure gaseous oxygen in the cryogenic separation, liquid oxygen extracted from the rectifier is compressed using a pump and is then heated in a heat exchanger to evaporate the liquid oxygen. As an advantage of this method, compression costs can be significantly reduced compared to compression of gaseous oxygen.
Raw air contains trace amounts of impurities, such as hydrocarbons, e.g., methane, ethane, ethylene, acetylene, propane, propylene, butane, butene, and pentane; carbon dioxide; and nitrogen oxides, in addition to major components, such as nitrogen, oxygen, and argon. Since such impurities have higher boiling points than those of nitrogen and oxygen and are less volatile, these are called heavy impurities. These heavy impurities are dissolved in liquid oxygen having lower volatility than that of nitrogen. Since the heavy impurities have higher boiling points and are less volatile compared to oxygen, these are concentrated in the liquid oxygen as evaporation of the liquid oxygen proceeds in the heat exchanger, and is precipitated in an oxygen channel in the heat exchanger as a solid phase or a liquid phase when the concentration exceeds the solubility to liquid oxygen. The precipitated heavy impurities will readily react with oxygen in the heat exchanger and clogs the oxygen channel. As a result, performance of the heat exchanger and thus overall performance of the apparatus will be deteriorated.
The following conventional means are disclosed for solving such problems.
Japanese Unexamined Patent Application Publication No. 7-174460 discloses extraction of a major fraction of liquid oxygen from a liquid phase having a relatively low heavy-impurity concentration at a second-bottom stage right above the lowermost stage in a lower pressure distillation tower. Moreover, a small fraction of liquid oxygen is extracted from the lowermost stage containing the largest amount of impurities. The extracted liquid oxygen is compressed to a pressure, which is equal to or higher than the final supply pressure, to raise the boiling point of oxygen, and is fed into a heat exchanger to raise the vapor pressure of heavy impurities contained in the liquid oxygen. Evaporation of the heavy impurities is thereby facilitated in the heat exchanger and the heavy impurities are not accumulated in the heat exchanger.
Japanese Unexamined Patent Application Publication No. 8-61843 discloses a method including a recycle flow for removing heavy impurities. The recycle flow means the following gas flow. A liquid having an enriched oxygen content of approximately 40% and containing concentrated heavy impurities is extracted from the bottom of a higher pressure rectifier and is sufficiently compressed so that the heavy impurities are evaporated in a heat exchanger. The pressure of the residual air is reduced and then the air is allowed to converge to raw air. The converged air flow is supplied to a preliminary purification unit to remove the heavy impurities.
These methods, however, still have the following problems. In the former method, the liquid oxygen extracted from the second-bottom stage contains a low concentration of heavy impurities. Thus, this method is not a basic countermeasure to precipitation of heavy impurities. When the system is continuously operated for long periods, for example, a year, the heavy impurities will be significantly precipitated in the heat exchanger. Since the system has two oxygen channels, the facility and operational costs are increased due to use of expensive apparatuses, such as liquid oxygen pumps, and a complicated overall processes.
The latter method also requires additional apparatuses such as liquid oxygen pumps for the cycle reflow. Thus, this method requires high facility and operational costs due to a complicated system and a complicated operation. Accordingly, this method is also not a basic countermeasure.